1. Field of the Invention
The invention relates to a method for the use of sorbents for the treatment of effluent gases, and more specifically, this invention relates to a method for the utilization of metal sorbents for the removal of mercury (Hg) from high temperature gases generated from sources such as coal gasifiers, coal-fired electrical generating plants, and ore smelters.
2. Background of the Invention
Gasification is an important strategy for increasing the utilization of abundant domestic coal resources. Rather than burning coal directly, coal gasification reacts coal with steam and carefully controlled amounts of air or oxygen under high temperatures and pressures to break apart the chemical bonds in coal to form a gaseous mixture, typically comprising hydrogen and carbon monoxide. The Department of Energy (DOE) envisions increased use of gasification in the United States during the next twenty years. As such, the DOE will strive for a goal of near-zero emissions of pollutants.
There are increasing concerns that air emissions from coal utilization, including gasification processes, will add mercury to the environment. The United States Environmental Protection Agency (EPA) intends to regulate coal-utilizing (gasifiers and combustors) facilities for mercury emissions. The Clear Skies Initiative, proposed in February 2002, would dramatically limit the emissions of mercury from all coal-utilizing facilities, including electrical generating plants.
There has been little progress in removing of mercury from high temperature gas streams. As such, nearly all of the mercury in the coal ends up in the flue gases (˜1 part per billion (ppb) concentration) and fuel gases (˜3 ppb concentration). Many sorbents' capacities for the adsorption of mercury is limited as shown in Table 1 infra.
Adsorption of mercury on a sorbent generally decreases as temperature increases. This is primarily true when the mechanism of removal is physical adsorption. Recent studies have suggested that carbon-based sorbents are unsuited for mercury capture from gas at temperatures higher than about 205° C. (400° F.). This is shown in: G. P. Reed, A. Ergudenler, J. R. Grace, A. P. Watkinson, A. A. Herod, D. Dugwell, and R. Kandiyoti, Fuel, 79, 1077–1087 (2000); 80, 623–634 (2001). Also, carbon lacks regeneration capabilities and chemical resistance.
Iridium (Ir) has been used as a trap for an ultraviolet detector for trace mercury determinations in water. This is shown in: H. W. Sinemus and H. H. Stabel, Spectrochimica Acta, 48B (14) 1719–1722 (1993). The iridium trap was regenerated in a two-step heating process. Heating at 925° C. (1700° F.) desorbed the mercury and heating at 1980° C. (3600° F.) removed any adsorbed arsenic (As) or selenium (Se) to restore the iridium trap to full efficiency.
TABLE 1Sorbent Capacities in Nitrogena,bSorbentCapacity (mg Hg/g sorbent)Temperature (° F.)MoS20.50400PbSe0.00400MnO2—Al2O30.14400Cr2O3—Al2O30.99400NiO—Al2O30.03400CuO—Al2O30.05400Co2O3—Al2O32.00400SiO20.00400amg = milligrams; g = grams.bThe absorption conditions comprised a gas composition of 270 parts per billion (ppb) mercury in N2; a flow-rate of 60 ml/min; a packed-bed reactor with an outer diameter of ¼ inch and an inner diameter of ⅙ inch. (The 270 ppb mercury concentration is similar to that found in incinerator effluent gases.)Ten mg of sorbent were used for each run. The length of sorbent exposure to the mercury/nitrogen gas mixture was 350 minutes (mins). The maximum effective capacity giving 100% Hg capture is 5.5 mg Hg/g sorbent.
Thermal desorption of surface-bound mercury from iridium foil has been shown to occur at between 165° C. (329° F.) and 190° C. (374° F.). Slower desorption of mercury from an iridium lattice in solid solution has been shown at temperatures between 190° C. (375° F.) and 500° C. (932° F.). This is demonstrated in: F. L. Fertonani, E. Milare, A. V. Benedetti, and M. Ionashiro, Journal of Thermal Analysis and Calorimetry, 67, 403–409 (2002).
U.S. Pat. No. 6,576,092 awarded to Granite, et al., on Jun. 10, 2003 discloses a method for the removal of mercury, the method comprising irradiating the mercury with UV light.
U.S. Pat. No. 6,521,021 awarded to Pennline, et al., on Feb. 18, 2003 discloses a system and method for the removal of mercury from the flue gas of a coal-fired power plant. A thermally activated sorbent is produced in situ at the power plant.
None of the aforementioned patents provides a high-capacity sorbent for mercury removal at temperatures greater than 205° C. (400° F.).
None of the aforementioned prior art provides a means for adsorbing mercury at temperatures greater than 205° C. (400° F.).
A need exists for a process to capture mercury from either flue gas or fuel gas (syngas) at high temperatures (i.e., greater than 200° C.). The process should incorporate a means for removing the target metal at ambient and above-ambient temperatures. The means should be comprised of readily-available materials. Further, the process should not require specialized training or equipment.